I read that letter about the white cross as well but unfortunately no such quest exists. Bethesda left the perfect set up for an anti-thieves guild quest but they never put it in, hopefully someone makes use of this. I also found it annoying how hostile the people of Skyrim were to me for no reason, I traveled the land helping people left and right yet everyone still talked to me like I was garbage. While most of the NPC's in this game are really horrible I was able to find one follower that was actually friendly which definitely improved the game for me. It is nice to travel the lands with someone who actually respects you at your side, having Derkeethus take out a few bandits from the shadows with his bow before I even reach them is not only helpful but also adds a feeling of comradery to the game. The land of Skyrim seems significantly less hostile when I have Derkeethus commentating on the amazing view when I reach the top of a tower or humming that little tune he does whenever I remain idle for too long. Skyrim definitely needs more friendly NPC's because I am sick of people like Maven and her family walking all over me and everybody else.